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Understanding vision
Seeing isn't just about the eyes. It begins in the brain.
A guide for families, adults and professionals to vision processing, cerebral visual impairment (CVI) and functional vision.
Vision skills build on each other like a pyramid.
An eye test checks the base.
The brain does everything above it, and when a lower layer is hard work, every layer above it is harder too.
Based on Warren's hierarchy of visual perceptual skills (Warren, 1993).
Vision is one of the brain's biggest jobs
Large parts of the brain work together to make sense of what we see. It happens all day, every day, mostly without us noticing. We use vision to:
Move
Judging steps, gaps and where things are.
Connect
Recognising faces and reading expressions.
Learn
Exploring, copying and making sense of new things.
Communicate
Using pictures, symbols, gestures and AAC.
Regulate
Feeling calm, or overwhelmed, in busy spaces.
So when vision processing is hard work, it can affect much more than seeing.
How seeing works
Think of it like taking a photo.
The eyes are the camera
They capture the picture.
The brain is the computer
It makes sense of the picture: what it is, where it is, and what to do next.
What is cerebral visual impairment (CVI)?
For some people, the camera works well, but the computer processes the picture differently. When the brain is the main reason someone sees the world differently, it's called cerebral visual impairment (CVI). CVI is the most common cause of visual impairment in children in developed countries, and it can also affect adults after a stroke or brain injury.
Others have both an eye condition and differences in how the brain processes vision. And because CVI is about the brain, a normal eye test doesn't rule it out.
Why vision can change from day to day
People who process vision differently often find their vision changes through the day. They might find a toy easily in the morning and not see it at all by the afternoon.
Think of a phone with too many apps open. Everything still works, but it slows down, freezes or drops things. When the brain is tired, unwell or busy managing other things, vision is often one of the first things to slip.
This is why one moment doesn't tell the whole story, and why we see people more than once and in different places.
Things that can affect vision on the day
Tiredness
Unfamiliar settings
Noise and busy places
Seizures
Illness or pain
Hunger
Looking and listening at the same time
Medication changes
Morning
Rested brain. Finds the toy on the shelf straight away.
Afternoon
Tired brain. The same toy on the same shelf is hard to spot.
When vision gets missed
Some people are more likely to process vision differently, including people who:
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were born early (premature)
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are autistic
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live with cerebral palsy
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live with epilepsy
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have hydrocephalus
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have a genetic difference, such as Down syndrome
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have had a brain injury or stroke
Processing vision differently isn't a flaw. It's part of how that person's brain takes in the world, and understanding it means the people around them can meet them where they are.
When someone already has a diagnosis, it's easy for everything to be put down to that diagnosis. Responses that are actually vision-related can be read as sensory, behavioural or attention needs, and the vision side gets missed.
Things that can be vision-related, but often aren't recognised as vision
Looking away when reaching for something
Turning the head to look, or looking from the side
Meltdowns or shutting down in busy, crowded places
Staring at lights, or being drawn to certain colours
Not recognising familiar people until they speak
Tripping on steps, kerbs or changes in flooring
Seeming to ignore things on one side
Tiring quickly with looking tasks
What is functional vision?
Functional vision is how we use our eyes and brain together in everyday life. Two people with the same diagnosis can use their vision in very different ways. It tends to show up in three parts of everyday life.
Looking and learning
Knowing where to look
Finding what matters in a busy space and tuning out the rest.
Reading and learning
Following words along a line, and recognising letters and numbers.
Visual curiosity
Showing interest in new sights, patterns or movement.
People and places
Recognising people and things
Spotting a parent at pick-up, or a favourite cup on a full shelf.
Reading faces
Noticing a smile, a frown or surprise.
Busy or bright places
Glare, dim light, patterned carpet or a crowded shopping centre.
Moving and doing
Moving around safely
Noticing a step, judging a kerb, weaving through the playground.
Hand–eye coordination
Reaching for a toy, catching a ball, drawing or using a spoon.
Vision with the other senses
Looking while listening, or watching while reaching.
Getting ready for an assessment
You don't need to prepare anything special. A few small things can help us make the most of our time together.
Think about your goals. What would make the biggest difference day to day?
Gather what you have. Recent eye test results, reports, school feedback or therapy notes.
Notice what is easy and what is tricky. Small everyday moments are often the most useful.
Have familiar things nearby. A favourite toy, glasses or a communication device.
Let them be themselves. Tired, shy, busy or needing breaks is all fine. That's real life, and it's what we want to see.